Silane coupling agents have been known to improve the mechanical properties of filled thermoseting and thermoplastic resins since the late 1940's. These low molecular weight compounds are believed to form chemical links between filler particles and polymer molecules, and as such, they must incorporate functional groups capable of reacting, or at least associating, with filler and resin alike. Although use of various silanes known in the art does greatly promote adhesion between thermoplastic polymers and substrates such as mineral fillers, exposure of these composites to water severely limits retention of the improved adhesion. Thus, for example, a moist environment can induce a gradual deterioration of the flexural strength of composites filled with silane-treated reinforcing fibers, and there is still need for improvement. Furthermore, when such fiber filled polymers are subjected to high shear rates, as in an injection molding operation, there is a tendency to destroy some of the covalent bonding (or any associative structure) formed between the coupling agent and the polymer. This also detracts from ultimate physical properties of the composite. There is thus a need for coupling agent which forms strong bonds or associations between itself and the polymer under ordinary conditions, which bonds become highly mobile at the elevated temperatues and shear rates encountered during injection molding. Even more desirable would be the availability of such a silane coupling agent which additionally imparted bond durability when challenged by conditions of high moisture.